1. Field of the Invention
The present invention relates to antennas for vehicles, and more particularly to a fixture for mounting an ultra high frequency grounded type antenna to a vehicle body.
2. Prior Art
Today, car telephones are used in many automobiles. There are two types of antennas used for car telephones. One type is a non-grounded antenna and the other is a grounded antenna.
Non-grounded antennas are usually not affected by external forces because of their impedance characteristics. Accordingly, they are widely used in car telephones. Since a non-grounded antenna does not need to be grounded, it is not necessary to drill a hole in the car body for mounting the antenna. Thus, the antenna is frequently mounted on the fender, trunk lid, bumper, etc. of the car. However, the non-grounded antenna has a complicated structure and is high in manufacturing cost.
On the other hand, the grounded antenna is relatively simple in structure and for this reason can be manufactured at low cost. Thus, it is desirable that the grounded antenna be used more frequently.
FIG. 5 shows an example of a conventional grounded antenna. In the Figure, the reference numeral 1 is an ultra high frequency antenna element. The antenna element 1 is connected to core 5a of a coaxial cable 5 through antenna base 2, a joint 3, and a contact pin 4. A conductive body 7 which coaxially surrounds the contact pin 4 through an insulator 6 is fixed and grounded on the vehicle body 9. Between the conductive body 7 and the vehicle body 9 is provided a grounding mount 8. The conductive body 7 is also connected to a shield cable 5b of the coaxial cable 5.
However this conventional antenna system has some problems since the grounded antenna requires a mounting hole on the vehicle body, the antenna mounting job is complicated. Also, such a hole could decrease the strength of the vehicle body. Thus, users are often reluctant to use a gounded antenna.
Furthermore, as shown by the dotted lines in FIG. 6(a), the grounded antenna must be grounded within the shortest possible distance between the grounding point 11 of a feeding section 10 and the grounding point 12 of the vehicle body 9. Practically, however, as shown by the solid line in FIG. 6(a), it is more likely that a lead section 13, which has the electric length l, is formed in the grounding circuit. When there is a lead section as shown in the Figure, the antenna output is split, by the impedance of the lead section, decreasing the gain and the antenna sensitivity.
In particular, if the impedance of the lead section 13 is ZL, and the impedance of the antenna is ZA, the equivalent circuit can be shown as FIG. 6(b) during the no-load period, and it can be shown as FIG. 6(c) during the load period such as when a transmission and reception device is connected to the antenna. As shown in FIGS. 6(b) and (c), ZL and ZA are always connected in series to the circuit. However, although the impedance ZL of the lead section 13 is zero when the electric length l equals zero, actually l does not become zero. Thus, the fact is that the impedance ZA of the antenna always has a certain value. As a result, the antenna output is split by the impedance ZL and the impedance ZA, resulting in loss of sensitivity loss.